Golf ball

ABSTRACT

The present invention provides a golf ball that achieves a low spin rate on driver shots to thereby improve flight distance performance, and also has improved durability, especially at high clubhead speeds. The present invention relates to a golf ball including a core, at least one middle layer covering the core, and a cover covering the middle layer, wherein at least one piece or layer of the middle layer includes a material for a middle layer that includes a thermoplastic polyurethane with a slab hardness of 65 to 80 in Shore D hardness and a ratio of upper yield stress (MPa) to lower yield stress (MPa) of not more than 1.60.

TECHNICAL FIELD

The present invention relates to a golf ball.

BACKGROUND ART

Golf balls that fly longer distances on driver shots have been desired.In order to enhance this flight distance performance, golf balls havinga multiple layer structure and improved materials for golf balls arebeing developed. One of the techniques to increase flight distance is todesign a golf ball to have an outer-hard/inner-soft structure. Forexample, a golf ball is proposed which includes a highly rigid, hardmiddle layer to enable the golf ball itself to have anouter-hard/inner-soft structure and thereby to achieve a low spin rateon driver shots.

Various kinds of ionomer resins are generally used as materials forproviding high rigidity to middle layers of golf balls to achieve lowspin rates. Unfortunately, however, the use of such a resin reducesdurability due to high rigidity.

Patent Literature 1 discloses a golf ball with excellent flight distanceperformance and the like, which includes a middle layer containing, as amain component, a thermoplastic resin other than ionomer resins, such asa thermoplastic polyurethane elastomer, and having a certain thicknessand a certain Shore D hardness. Patent Literatures 2 and 3 disclose golfballs with longer flight distances which include a middle layercontaining a thermoplastic polyurethane elastomer and the like andhaving a certain bending rigidity.

Unfortunately, the above-mentioned golf balls with a middle layercontaining a thermoplastic polyurethane elastomer leave room forimprovement as they are not sufficiently durable at high clubheadspeeds.

CITATION LIST Patent Literature

Patent Literature 1: JP 2004-8404 A

Patent Literature 2: JP 2004-187991 A

Patent Literature 3: JP 2004-242850 A

SUMMARY OF INVENTION Technical Problem

The present invention aims to solve the above problems and to provide agolf ball that achieves a low spin rate on driver shots to therebyimprove flight distance performance, and also has improved durability,especially at high clubhead speeds.

Solution to Problem

The present invention relates to a golf ball, including a core, at leastone middle layer covering the core, and a cover covering the middlelayer, wherein at least one piece or layer of the middle layer includesa material for a middle layer that includes a thermoplastic polyurethanewith a slab hardness of 65 to 80 in Shore D hardness and a ratio ofupper yield stress (MPa) to lower yield stress (MPa) of not more than1.60.

The thermoplastic polyurethane preferably has an upper yield stress ofnot less than 15 MPa and a lower yield stress of not less than 10 MPa.Moreover, the thermoplastic polyurethane preferably has a breakingstress (MPa) of not less than 25 MPa.

The thermoplastic polyurethane preferably has a ratio of breaking stress(MPa) to slab hardness (Shore D hardness) of not less than 0.70.Moreover, the thermoplastic polyurethane preferably has a bendingrigidity of 250 to 4000 MPa.

Preferably, the middle layer has a thickness of 0.5 to 2.0 mm.Preferably, the middle layer has a surface hardness of 65 to 80 in ShoreD hardness. Preferably, a difference (Hm−Hs) between a surface hardness(Hm) of the middle layer and a surface hardness (Hs) of the core is 3 to25.

Preferably, the cover includes a thermoplastic polyurethane with a slabhardness of not more than 50 in Shore D hardness, and has a thickness of0.3 to 1.5 mm.

Advantageous Effects of Invention

The present invention provides a golf ball including a core, at leastone middle layer covering the core, and a cover covering the middlelayer, wherein at least one piece or layer of the middle layer includesa material for a middle layer that includes a thermoplastic polyurethanewith a slab hardness of 65 to 80 in Shore D hardness and a ratio ofupper yield stress (MPa) to lower yield stress (MPa) of not more than1.60. Thus, the golf ball not only achieves a low spin rate on drivershots to thereby improve flight distance performance, but also hasimproved durability, especially at high clubhead speeds.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a stress-strain curve 1 of a thermoplastic polyurethane(Elastollan 1174D).

FIG. 2 shows a stress-strain curve 2 of a thermoplastic polyurethane.

DESCRIPTION OF EMBODIMENTS

The golf ball of the present invention includes a core, at least onemiddle layer covering the core, and a cover covering the middle layer,wherein at least one piece or layer of the middle layer includes amaterial for a middle layer that includes a thermoplastic polyurethanewith a slab hardness of 65 to 80 in Shore D hardness and a ratio ofupper yield stress (MPa) to lower yield stress (MPa) of not more than1.60.

By using the thermoplastic polyurethane having a specific slab hardnessand a specific ratio of upper yield stress to lower yield stress as aresin component of a middle layer, it is possible to not only achieve alow spin rate on driver shots to thereby increase flight distance, butalso to improve durability, especially at high clubhead speeds. It isalso possible to ensure a soft ball with a large compressive deformationto thereby give good shot feeling.

First, the material for a middle layer will be explained.

A middle layer is formed of a material for a middle layer whichincludes, as a resin component, a thermoplastic polyurethane(thermoplastic polyurethane elastomer) that has a specific slab hardnessand a specific ratio of upper yield stress to lower yield stress ([upperyield stress]/[lower yield stress]).

The slab hardness of the thermoplastic polyurethane is not less than 65,preferably not less than 67, and more preferably not less than 69 inShore D hardness, whereas it is not more than 80, preferably not morethan 78, and more preferably not more than 76 in Shore D hardness. Theuse of such a high hardness thermoplastic polyurethane enables a lowspin rate on driver shots, thus improving flight distance performance.It should be noted that the slab hardness (Shore D hardness) can bemeasured by the method mentioned later.

The thermoplastic polyurethane has a ratio of upper yield stress (MPa)to lower yield stress (MPa) of not more than 1.60, preferably not morethan 1.50, and more preferably not more than 1.40. In the case that thedifference between the upper and lower yield stresses at a yield pointis large, large distortion is likely to occur when an impact exceedingthe yield point is applied. The deformation further causes a strain, andas a result durability is likely to deteriorate. The difference betweenthe upper and lower yield stresses at a yield point is preferably small.The closer to 1.00 the ratio of upper yield stress (MPa) to lower yieldstress (MPa) is, the less likely the distortion due to impact is tooccur, and, in turn, the better the durability tends to be.

The thermoplastic polyurethane preferably has an upper yield stress ofnot less than 15 MPa, more preferably not less than 18 MPa, and stillmore preferably not less than 21 MPa. If the upper yield stress is toolow, a small stress tends to easily exceed the yield point, which tendsto result in more deformation and therefore poor resistance to impact.The upper limit of the upper yield stress is not particularly limited,and the upper yield stress is preferably not more than 60 MPa, morepreferably not more than 55 MPa, and still more preferably not more than50 MPa.

The thermoplastic polyurethane preferably has a lower yield stress ofnot less than 10 MPa, more preferably not less than 13 MPa, and stillmore preferably not less than 16 MPa. If the lower yield stress is toolow, the deformation at a point exceeding the upper yield point tends tobecome greater so that the resistance to impact is likely todeteriorate. The upper limit of the lower yield stress is notparticularly limited, and the lower yield stress is preferably not morethan 50 MPa, more preferably not more than 45 MPa, and still morepreferably not more than 40 MPa.

In the present invention, the upper yield stress and the lower yieldstress are determined from a stress-strain curve obtained by a tensiletest performed in accordance with ISO 527-1, as shown in FIG. 1 or 2,for example. Specifically, they are determined by the method mentionedlater. If an upper yield stress and a lower yield stress do not clearlyappear in an obtained stress-strain curve as shown in FIG. 2, the upperyield stress is regarded to be equal to the lower yield stress, whichindicates that the ratio of upper yield stress to lower yield stress is1.00.

The thermoplastic polyurethane preferably has a breaking stress (MPa) ofnot less than 25 MPa, more preferably not less than 28 MPa, and stillmore preferably not less than 30 MPa. A breaking stress of not less than25 MPa ensures excellent durability. The upper limit of the breakingstress is not particularly limited, and the breaking stress ispreferably not more than 65 MPa, more preferably not more than 60 MPa,and still more preferably not more than 55 MPa.

The thermoplastic polyurethane preferably has a ratio of breaking stress(MPa) to slab hardness (Shore D hardness) of not less than 0.70, morepreferably not less than 0.72, and still more preferably not less than0.74. A ratio of not less than 0.70 ensures excellent durability. Themaximum ratio of breaking stress (MPa) to slab hardness (Shore Dhardness) is not particularly limited. In view of the need of highhardness for improvement in flight distance performance and ofsimultaneous achievement of durability and flight distance performance,the ratio is preferably not more than 0.88, more preferably not morethan 0.86, and still more preferably not more than 0.84. It should benoted that the breaking stress and the slab hardness (Shore D hardness)can be measured by the methods mentioned later.

The thermoplastic polyurethane preferably has a bending rigidity of notless than 250 MPa, more preferably not less than 280 MPa, and still morepreferably not less than 300 MPa. The bending rigidity is preferably notmore than 4000 MPa, more, preferably not more than 3600 MPa, and stillmore preferably not more than 3200 MPa. The use of such a high rigiditythermoplastic polyurethane enables a low spin rate on driver shots andthus improves flight distance performance. It should be noted that thebending rigidity herein refers to a value determined in accordance withISO 178.

The thermoplastic polyurethane preferably has a number average molecularweight of not less than 20,000, more preferably not less than 30,000,and still more preferably not less than 40,000, whereas it preferablyhas a number average molecular weight of not more than 200,000, morepreferably not more than 150,000, and still more preferably not morethan 100,000. With a number average molecular weight of not less than20,000, the resulting golf ball can have further improved scratchresistance. With a number average molecular weight of not more than200,000, conversely, the thermoplastic polyurethane has good fluidityand good moldability. It should be noted that the number averagemolecular weight (Mn) can be determined by gel permeation chromatography(GPC) relative to polystyrene standards.

The thermoplastic polyurethane with the certain slab hardness and thecertain ratio of upper yield stress to lower yield stress may furtherhave a plurality of urethane bonds within the molecule and thus exhibitthermoplasticity. Examples of such thermoplastic polyurethanes includeproducts obtained by reacting a polyisocyanate with a polyol to formurethane bonds within the molecule, optionally followed by furthercausing a chain extension reaction with a low-molecular-weight polyol orpolyamine, or the like.

The polyisocyanate component forming the thermoplastic polyurethane maybe one that has two or more isocyanate groups. Examples thereof includearomatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, mixtures of 2,4-toluene diisocyanate and 2,6-toluenediisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI),1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethyl xylylene diisocyanate(TMXDI), and para-phenylene diisocyanate (PPDI); and alicyclic oraliphatic polyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate(H₁₂MDI), hydrogenated xylylene diisocyanate (H₆XDI), hexamethylenediisocyanate (HDI), isophorone diisocyanate (IPDI), and norbornenediisocyanate (NBDI). These may be used alone, or two or more of thesemay be used in combination. In view of enhancing flight distanceperformance and durability, xylylene diisocyanate (XDI) or hexamethylenediisocyanate (HDI) is preferred among these polyisocyanate components,and 4,4′-diphenylmethane diisocyanate (MDI) is more preferred.

The polyol component forming the thermoplastic polyurethane may be onethat has a plurality of hydroxy groups, such as, for example, alow-molecular-weight polyol or a high-molecular-weight polyol. Examplesof the low molecular-weight polyols include diols such as ethyleneglycol, diethylene glycol, triethylene glycol, propanediol (e.g.1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol), dipropyleneglycol, butanediol (e.g. 1,2-butanediol, 1,3-butanediol, 1,4-butanediol,2,3-butanediol, 2,3-dimethyl-2,3-butanediol), neopentyl glycol,pentanediol, hexanediol, heptanediol, octanediol,1,6-cyclohexanedimethylol, aniline diols, and bisphenol A diols; triolssuch as glycerin, trimethylol propane, and hexanetriol; and tetraols andhexanols, such as pentaerythritol and sorbitol. Examples of thehigh-molecular-weight polyols include polyether polyols such aspolyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), andpolyoxytetramethylene glycol (PTMG); condensed polyester polyols such aspolyethylene adipate (PEA), polybutylene adipate (PBA), orpolyhexamethylene adipate (PHMA); lactone polyester polyols such aspoly-ε-caprolactone (PCL); polycarbonate polyols such aspolyhexamethylene carbonate; and acrylic polyols. These may be usedalone, or two or more of these may be used in combination.

The high-molecular-weight polyol has any number average molecular weightalthough the number average molecular weight is preferably not less than400, and more preferably not less than 1,000. The use of ahigh-molecular-weight polyol having an excessively low number averagemolecular weight may produce a hard polyurethane, resulting in golfballs with reduced shot feeling. The upper limit of the number averagemolecular weight is preferably not more than 10,000 and more preferablynot more than 8,000.

The polyamine optionally forming the thermoplastic polyurethane may beone that has two or more amino groups. Examples of the polyaminesinclude aliphatic polyamines such as ethylenediamine, propylenediamine,butylenediamine, and hexamethylenediamine; alicyclic polyamines such asisophoronediamine and piperazine; and aromatic polyamines.

The aromatic polyamine may be one that has two or more amino groupsdirectly or indirectly bonded to an aromatic ring. Herein, the term“indirectly bonded” means that the amino group is bonded to the aromaticring via, for example, a lower alkylene group. The aromatic polyaminemay be, for example, a monocyclic aromatic polyamine having two or moreamino groups bonded to one aromatic ring or a polycyclic aromaticpolyamine having two or more aminophenyl groups in which at least oneamino group is bonded to one aromatic ring.

Examples of the monocyclic aromatic polyamines include a kind ofpolyamines in which amino groups are directly bonded to an aromaticring, such as phenylenediamine, toluenediamine, diethyltoluenediamine,and dimethylthiotoluenediamine; and a kind of polyamines in which aminogroups are bonded to an aromatic ring via a lower alkylene group, suchas xylylenediamine. The polycyclic aromatic polyamine may be apoly(aminobenzene) having two or more aminophenyl groups directly bondedto each other, or may be one having two or more aminophenyl groupsbonded via a lower alkylene group or an alkylene oxide group. In view ofreactivity, 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline) ispreferred among these polyamines, and4,4′-methylene-bis-(2,6-diethylaniline) is more preferred.

Structural embodiments of the thermoplastic polyurethane include: anembodiment in which the thermoplastic polyurethane is formed from apolyisocyanate component and a high-molecular-weight polyol component;an embodiment in which the thermoplastic polyurethane is formed from apolyisocyanate component, a high-molecular-weight polyol component, anda low-molecular-weight polyol component; an embodiment in which thethermoplastic polyurethane is formed from a polyisocyanate component, ahigh-molecular-weight polyol component, a low-molecular-weight polyolcomponent, and a polyamine component; and an embodiment in which thethermoplastic polyurethane is formed from a polyisocyanate component, ahigh-molecular-weight polyol component, and a polyamine component.

Specific examples of the thermoplastic polyurethane include Elastollan(registered trademark) 1164D and 1174D, produced by BASF Japan Ltd., andMiractran E568 and E574 produced by Nippon Miractran Co., Ltd.

The resin component forming the middle layer preferably contains thethermoplastic polyurethane in an amount of not less than 50% by mass,more preferably not less than 65% by mass, and still more preferably notless than 80% by mass. The amount of the thermoplastic polyurethane maybe 100% by mass. When the resin component contains the thermoplasticpolyurethane in an amount of not less than 50% by mass, i.e., containsthe thermoplastic polyurethane as a main component, it is more effectivein enhancing low spin and in improving durability.

The material for a middle layer may contain a resin component other thanthe thermoplastic polyurethane, such as various ionomer resins, as longas it does not impair the effects of the present invention. Moreover,other additives may be added to the extent that they do not impair theeffects of the present invention. Examples of the additives include:pigments such as white pigments (e.g. titanium oxide), blue pigments,and red pigments; weighting agents such as calcium carbonate or bariumsulfate; dispersants; antioxidants; ultraviolet absorbents; lightstabilizers; fluorescent materials and fluorescent brighteners.

In cases where the middle layer contains materials other than thethermoplastic polyurethane, the material for a middle layer, includingall the materials forming the middle layer, desirably has a slabhardness (Shore D), a ratio of upper yield stress to lower yield stress,an upper yield stress, a lower yield stress, a breaking stress (MPa), aratio of breaking stress (MPa) to slab hardness (Shore D hardness), anda bending rigidity, all falling within the ranges as defined for thethermoplastic polyurethane. Specifically, these properties can beadjusted to desired values by using the thermoplastic polyurethane as amain component and further appropriately selecting and using othercomponents to the extent that they do not greatly affect the properties.Each property can be measured in the same manner as mentioned above.

The following will explain the golf ball.

The golf ball of the present invention includes a core, at least onemiddle layer covering the core, and a cover covering the middle layer.At least one piece or layer of the middle layer is formed of thematerial for a middle layer including the thermoplastic polyurethane.

Any core may be used in the present invention. The core may be, forexample, a core consisting of a center, or a multilayered core (e.g.two-layered core) consisting of a center and at least one surroundinglayer that covers the center.

The center may be formed of a conventional rubber composition(hereinafter, also referred to simply as “rubber composition for acenter”) or a resin composition. For example, the center may be formedby hot-pressing a rubber composition that includes a base rubber, acrosslinking initiator, a co-crosslinking agent, and a filler. The baseresin of the resin composition may be a thermoplastic resin such asionomer resin, thermoplastic olefin copolymer, thermoplasticpolyurethane resin, thermoplastic polyamide resin, thermoplastic styreneresin, thermoplastic polyester resin, or thermoplastic acrylic resin.

The base rubber may be natural rubber and/or a synthetic rubber, andexamples thereof include polybutadiene rubber, natural rubber,polyisoprene rubber, styrene polybutadiene rubber, andethylene-propylene-diene rubber (EPDM). Preferred among them, inparticular, is high-cis polybutadiene having cis bonds, whichadvantageously improve resilience, in an amount of 40% by mass or more,preferably 70% by mass or more, and more preferably 90% by mass or more.

The crosslinking initiator is intended to crosslink the base rubbercomponent. The crosslinking initiator is suitably an organic peroxide.Specific examples of the organic peroxides include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide,preferred among which is dicumyl peroxide. The amount of crosslinkinginitiator to be added is preferably not less than 0.1 parts by mass,more preferably not less than 0.3 parts by mass, and still morepreferably not less than 0.5 parts by mass, whereas it is preferably notmore than 3 parts by mass, more preferably not more than 2.8 parts bymass, and still more preferably not more than 2.5 parts by mass, per 100parts by mass of the base rubber. If the amount is less than 0.1 partsby mass, the core tends to become too soft and thus resilience tends tobe lowered. If the amount is more than 3 parts by mass, a larger amountof co-crosslinking agent is necessary for a proper hardness, which leadsto insufficient resilience.

Any co-crosslinking agent may be used as long as it functions tocrosslink rubber molecules by graft polymerization onto the molecularchains of the base rubber. Examples of the co-crosslinking agentsinclude α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms andmetal salts thereof, preferably acrylic acid, methacrylic acid, andmetal salts thereof. Examples of metals that can be used to form themetal salts include zinc, magnesium, calcium, aluminum, and sodium,preferred among which is zinc because it provides high resilience.

The amount of co-crosslinking agent to be added is preferably not lessthan 10 parts by mass, and more preferably not less than 15 parts bymass, whereas it is preferably not more than 50 parts by mass, and morepreferably not more than 45 parts by mass, per 100 parts by mass of thebase rubber. If the amount of co-crosslinking agent is less than 10parts by mass, a larger amount of crosslinking initiator is likely tonecessary for a proper hardness, which tends to reduce resilience.Conversely, if the amount is more than 50 parts by mass, the center maybecome so hard that shot feeling can be reduced.

The rubber composition for a center may optionally contain a fillermainly as a weighting agent to adjust the specific gravity of a golfball obtained as an end product in the range of 1.0 to 1.5. Examples ofthe fillers include inorganic fillers such as zinc oxide, bariumsulfate, calcium carbonate, magnesium oxide, tungsten powder, andmolybdenum powder. The amount of filler to be added is preferably notless than 0.5 part by mass, and more preferably not less than 1 part bymass, whereas it is preferably not more than 30 parts by mass, and morepreferably not more than 20 parts by mass, per 100 parts by mass of thebase rubber. If the amount is less than 0.5 parts by mass, it isdifficult to adjust the weight. If the amount is more than 30 parts bymass, the weight ratio of the rubber component is reduced, which tendsto lead to reduced resilience.

The rubber composition for a center may further appropriately contain anorganic sulfur compound, an antioxidant, a peptizing agent, and otheradditives in addition to the base rubber, the crosslinking initiator,the co-crosslinking agent and the filler.

The organic sulfur compound may suitably be a diphenyl disulfide.Examples of the diphenyl disulfides include: diphenyl disulfide;mono-substituted diphenyl disulfides such asbis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide,bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide,bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide andbis(4-cyanophenyl)disulfide; di-substituted diphenyl disulfides such asbis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide,bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide,bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide,and bis(2-cyano-5-bromophenyl)disulfide; tri-substituted diphenyldisulfides such as bis(2,4,6-trichlorophenyl)disulfide andbis(2-cyano-4-chloro-6-bromophenyl)disulfide; tetra-substituted diphenyldisulfides such as bis(2,3,5,6-tetra chlorophenyl)disulfide; andpenta-substituted diphenyl disulfides such asbis(2,3,4,5,6-pentachlorophenyl)disulfide andbis(2,3,4,5,6-pentabromophenyl)disulfide. These diphenyl disulfides canenhance resilience by somehow affecting the state of cure of rubbervulcanizates. Preferred among them are diphenyl disulfide andbis(pentabromophenyl)disulfide because the resulting golf ballsparticularly have high resilience. The amount of organic sulfur compoundto be added is preferably not less than 0.1 parts by mass, and morepreferably not less than 0.3 parts by mass, whereas it is preferably notmore than 5.0 parts by mass, and more preferably not more than 3.0 partsby mass, per 100 parts by mass of the base rubber.

The amount of antioxidant to be added preferably ranges from 0.1 to 1part by mass per 100 parts by mass of the base rubber. The amount ofpeptizing agent preferably ranges from 0.1 to 5 parts by mass per 100parts by mass of the base rubber.

The center may be prepared by mixing and kneading the rubber compositionand molding it in a die under any conditions. Preferably, for example,the rubber composition is heated at a temperature of 130 to 200° C. for10 to 60 minutes, or alternatively, is heated in two steps, i.e., at atemperature of 130 to 150° C. for 20 to 40 minutes, and subsequently ata temperature of 160 to 180° C. for 5 to 15 minutes.

The following will explain the surrounding layer forming themultilayered core, if used.

The surrounding layer is formed of a composition for a surroundinglayer, examples of which include: thermoplastic resins such as ionomerresins commercially available under the trade name “HIMILAN (registeredtrademark) (e.g. HIMILAN 1605, HIMILAN 1706)” from DuPont-MitsuiPolychemical, and under the trade name “SURLYN (registered trademark)(e.g. SURLYN 8140, SURLYN 9120)” from E.I. du Pont de Nemours andCompany; and thermoplastic elastomers such as thermoplastic polyamideelastomers commercially available under the trade name “PEBAX(registered trademark) (e.g. PEBAX 2533)” from Arkema, thermoplasticpolyester elastomers commercially available under the trade name “HYTREL(registered trademark) (e.g. HYTREL 3548, HYTREL 4047)” from DuPont-Toray Co., Ltd., thermoplastic polyurethane elastomers commerciallyavailable under the trade name “ELASTOLLAN (registered trademark) (e.g.ELASTOLLAN XNY97A)” from BASF Japan Ltd., and thermoplastic polystyreneelastomers commercially available under the trade name “RABALON(registered trademark)” from Mitsubishi Chemical Corporation, as well asrubber compositions as mentioned for the composition for a center. Thesethermoplastic resins and thermoplastic elastomers may be used alone, ortwo or more of them may be used in admixture.

The surrounding layer may be formed, for example, by covering the centerwith the composition for a surrounding layer. The method for forming thesurrounding layer is not particularly limited. For example, thesurrounding layer may be formed by molding the composition for asurrounding layer into hemispherical half-shells in advance, enclosingthe center with two pieces of half-shells, and pressing the assembly ata temperature of 130 to 170° C. for 1 to 5 minutes, or by injectionmolding the composition for a surrounding layer directly onto the centerto enclose the center.

The composition for a surrounding layer preferably has a slab hardnessof not less than 40, more preferably not less than 42, and still morepreferably not less than 43 in Shore D hardness, whereas the slabhardness is preferably not more than 70, more preferably not more than66, and still more preferably not more than 64 in Shore D hardness. Ifthe composition has a slab hardness of not less than 40, the resultinggolf ball has better resilience. If the composition has a slab hardnessof not more than 70, the resulting golf ball gives better shot feeling.The slab hardness of the composition for a surrounding layer can beadjusted by appropriately selecting a combination of the resincomponents or rubber compositions mentioned above.

The center preferably has a diameter of not less than 5.0 mm, morepreferably not less than 10.0 mm, whereas it preferably has a diameterof not more than 41.5 mm, more preferably not more than 41.0 mm, andstill more preferably not more than 40.5 mm. If the center has adiameter of not less than 5.0 mm, the function of the relatively softcenter is more exerted, thus further reducing the spin rate, especiallyon W #1 shots. Conversely, if the diameter thereof is not more than 41.5mm, then the surrounding layer, the middle layer, or the cover layer isnot too thin, and thus the function of each layer is more exerted.

In cases where the center has a diameter of 5.0 to 41.5 mm, the amountof compressive deformation (shrinkage of the center in the compressiondirection) when applying from an initial load of 98 N to a final load of1275 N is preferably not smaller than 4.0 mm, and more preferably notsmaller than 4.5 mm, whereas it is preferably not greater than 10.0 mm,and more preferably not greater than 8.0 mm. When the amount ofcompressive deformation is not smaller than 4.0 mm, then better shotfeeling is provided. When the amount of compressive deformation is notgreater than 10.0 mm, then better resilience is achieved.

The surrounding layer preferably has a thickness of not smaller than 3.0mm, more preferably not smaller than 5.0 mm, and still more preferablynot smaller than 7.0 mm. The thickness is also preferably not largerthan 17.0 mm, more preferably not larger than 15.0 mm, and still morepreferably not larger than 13.0 mm. The surrounding layer having athickness not smaller than the lower limit mentioned above provides alarger effect, thus further enhancing the effect of reducing spin on,for example, driver shots. When the surrounding layer has a thicknessnot larger than the upper limit mentioned above, then the core exhibitsa greater influence, thus making resilience much better.

In the case of a multilayered core including the center and at least onesurrounding layer covering the center, the multilayered core preferablyhas a diameter of not less than 32.0 mm, more preferably not less than34.0 mm, and still more preferably not less than 39.0 mm, whereas itpreferably has a diameter of not more than 41.5 mm, more preferably notmore than 41.0 mm, and still more preferably not more than 40.5 mm. Whenthe core has a diameter within the range mentioned above, the effect ofreducing spin on, for example, driver shots is further enhanced.

In cases where the core has a diameter of 32.0 to 41.5 mm, the amount ofcompressive deformation (shrinkage of the core in the compressiondirection) when applying from an initial load of 98 N to a final load of1275 N is preferably not smaller than 2.0 mm, more preferably notsmaller than 2.2 mm, and still more preferably not smaller than 2.3 mm,whereas it is preferably not greater than 4.5 mm, more preferably notgreater than 4.0 mm, and still more preferably not greater than 3.5 mm.When the amount of compressive deformation is not smaller than 2.0 mm,the effect of reducing spin on, for example, driver shots and shotfeeling are further improved. When the amount of compressive deformationis not greater than 4.5 mm, resilience becomes much better.

The difference (Hs−Ho) between the surface hardness (Hs) and thecenter-point hardness (Ho) of the core is preferably not less than 10,more preferably not less than 15, and still more preferably not lessthan 20 in Shore D hardness. When the surface of the core is harder thanthe center point thereof, a larger launch angle and reduced spin rateare achieved, so that flight distance is increased. The differencebetween the surface hardness and the center-point hardness of the coreis preferably not more than 55, more preferably not more than 50, andstill more preferably not more than 40 in Shore D hardness. If thedifference in hardness is too large, durability may be reduced.

Moreover, the center-point hardness (Ho) of the core is preferably notless than 20, more preferably not less than 27, and still morepreferably not less than 32 in Shore D hardness. When the center-pointhardness is not less than 20, the core is not too soft and thus canprovide good resilience. The center-point hardness (Ho) is preferablynot more than 70, more preferably not more than 65, and still morepreferably not more than 62 in Shore D hardness. When the center-pointhardness is not more than 70, the core is not too hard and thus can givea good shot feeling. The center-point hardness of the core herein refersto a hardness measured as follows: the core is divided into two equalparts, and the hardness is then measured at the center point of thecross section using a spring type Shore D hardness tester.

The core of the golf ball of the present invention preferably has asurface hardness (Hs) of not less than 45, more preferably not less than47, and still more preferably not less than 48 in Shore D hardness. Whenthe surface hardness is not less than 45, the core is not too soft andthus can provide good resilience. The core preferably has a surfacehardness (Hs) of not more than 65, more preferably not more than 63, andstill more preferably not more than 60 in Shore D hardness. If thesurface hardness is not more than 65, the difference in hardness fromthe middle layer can be increased, which further increases the effect ofreducing spin on driver shots.

The following will explain the at least one middle layer covering thecore.

A material for a middle layer containing the above-mentionedthermoplastic polyurethane is used in at least one piece or layer of themiddle layer.

The middle layer may be formed, for example, by covering the core withthe material for a middle layer. The method for forming the middle layeris not particularly limited. For example, the middle layer may be formedby molding the material for a middle layer into hemisphericalhalf-shells in advance, enclosing the core with two pieces ofhalf-shells, and pressing the assembly at a temperature of 130 to 170°C. for 1 to 5 minutes, or by injection molding the material for a middlelayer directly onto the core to enclose the core.

The middle layer formed of the material for a middle layer preferablyhas a thickness of not smaller than 0.5 mm, more preferably not smallerthan 0.6 mm, and still more preferably not smaller than 0.7 mm, whereasit preferably has a thickness of not larger than 2.0 mm, more preferablynot larger than 1.8 mm, and still more preferably not larger than 1.6mm. If the middle layer has a thickness smaller than 0.5 mm, durabilitymay be deteriorated due to the thin middle layer. If the middle layerhas a thickness larger than 2.0 mm, then the core has a smallerdiameter, which may result in low resilience.

The middle layer formed of the material for a middle layer preferablyhas a surface hardness (Hm) of not less than 65, more preferably notless than 66, and still more preferably not less than 67, whereas itpreferably has a surface hardness (Hm) of not more than 80, morepreferably not more than 78, and still more preferably not more than 76,in Shore D hardness. When the surface hardness is not less than 65, themiddle layer has high hardness and high rigidity and thus the effect ofreducing spin on, for example, driver shots is further enhanced. Whenthe surface hardness is not more than 80, the middle layer is not toohard and thus golf ball durability and shot feeling are furtherimproved.

The difference (Hm−Hs) between the surface hardness (Hm) of the middlelayer formed of the material for a middle layer and the surface hardness(Hs) of the core is preferably not less than 3, more preferably not lessthan 4, and still more preferably not less than 5, whereas it ispreferably not more than 25, more preferably not more than 18, and stillmore preferably not more than 16, in Shore D hardness. When thedifference (Hm−Hs) in surface hardness falls within the range mentionedabove, the spin rate is further reduced, so that flight distance isincreased.

Embodiments of combinations of the core and the at least one middlelayer include an embodiment in which the core is covered with one middlelayer; and an embodiment in which the core is covered with multiplepieces or layers of middle layer.

The core covered with the middle layer preferably has a spherical shape.This is because, if the shape of the middle layer formed is notspherical, the cover has an uneven thickness and thus partially hasreduced covering performance. Meanwhile, the core has a spherical shape,in general. The spherical core may be provided with elongatedprotrusion(s) to divide the surface of the spherical core, for example,to equally divide the surface of the spherical core. For example, in anembodiment in which the elongated protrusion(s) is provided, theelongated protrusion(s) is integrally formed with the surface of thesurrounding layer. In another embodiment, the surface of the sphericalcenter is provided with a surrounding layer in the form of elongatedprotrusion(s).

If the spherical core is regarded as the earth, for example, theelongated protrusion(s) is preferably provided along the equator and anymeridians that equally divide the surface of the spherical core. Forexample, in the case of dividing the surface of the spherical core into8 parts, elongated protrusions may be provided along the equator, anymeridian (0 degrees longitude) and the meridians at 90 degrees eastlongitude, at 90 degrees west longitude, and at 180 degrees east (west)longitude based on the meridian at 0 degrees longitude. When theelongated protrusion(s) is provided on the surface of the core, concaveportions separated by the elongated protrusion(s) are preferably filledby multiple middle layers, or a single middle layer that covers eachconcave portion, so that the covered core has a spherical shape. Theelongated protrusion may have any cross-sectional shape, and may have,for example, an arc shape, a substantially arc shape (for example, ashape in which a notch portion is formed at a part where the elongatedprotrusions intersect one another or are at right angles to each other),or the like.

Regarding the middle layer, in the case that the core is covered with asingle middle layer or multiple middle layers, at least one layer of themiddle layer(s) is formed of the material for a middle layer. In thecase that the concave portions separated by elongated protrusionsprovided on the surface of the core are filled by multiple pieces of themiddle layer, at least one piece of the multiple pieces of the middlelayer is formed of the material for a middle layer. In cases where thecore is covered with multiple pieces or layers of the middle layer, themiddle layer may include a middle layer formed of a material for amiddle layer different from the earlier mentioned material for a middlelayer, as long as it does not impair the effects of the presentinvention. In this case, the middle layer formed of the earliermentioned material for a middle layer is preferably placed in contactwith the cover. Preferably, all of the multiple pieces or layers of themiddle layer are formed of the material for a middle layer.

The other material for a middle layer different from the earliermentioned material for a middle layer may be, for example, a material asmentioned for the composition for a surrounding layer. The material mayadditionally contain a weighting agent (e.g. barium sulfate, tungsten),an antioxidant, a pigment and the like.

Examples of the resin component in a cover composition forming the coverinclude, in addition to polyurethane resins and conventional ionomerresins, thermoplastic polyamide elastomers commercially available underthe trade name “PEBAX (registered trademark) (e.g. PEBAX 2533)” fromArkema; thermoplastic polyester elastomers commercially available underthe trade name “HYTREL (registered trademark) (e.g. HYTREL 3548, HYTREL4047)” from Du Pont-Toray Co., Ltd.; and thermoplastic polystyreneelastomers commercially available under the trade name “RABALON(registered trademark)” from Mitsubishi Chemical Corporation. Theseresins used as the resin component may be used alone, or two or more ofthese may be used in combination. Preferred among these are polyurethaneresins.

The cover composition forming the cover of the golf ball of the presentinvention preferably contains a resin component that includes 50% bymass or more, more preferably 60% by mass or more, and still morepreferably 70% by mass or more of a polyurethane resin. Most preferably,the resin component of the cover composition consists only of apolyurethane resin.

The polyurethane resin may be any one that has a plurality of urethanebonds within the molecule. Examples thereof include products obtained byreacting a polyisocyanate component and a high-molecular-weight polyolcomponent to form urethane bonds within the molecule, optionallyfollowed by further causing a chain extension reaction with alow-molecular-weight polyol or polyamine, or the like.

The polyurethane resin preferably has a slab hardness of not higher than50, more preferably not higher than 40, and still more preferably nothigher than 35 in Shore D hardness. The use of a polyurethane resinhaving a low hardness improves spin performance on approach shots. Theslab hardness is preferably not lower than 10, and more preferably notlower than 15 in Shore D hardness. A polyurethane resin having a slabhardness of lower than 10 may lead to excessively high spin rates onapproach shots. Specific examples of the polyurethane resin includeELASTOLLAN (registered trademark) XNY75A, XNY80A, XNY83A, XNY85A,XNY97A, XNY90A, and ET880, produced by BASF Japan Ltd.

In the present invention, the cover may contain, in addition to theabove-mentioned resin component, any of the following additives:pigments such as titanium oxide, a blue pigments, and red pigments;weighting agents such as zinc oxide, calcium carbonate, and bariumsulfate; dispersants; antioxidants; ultraviolet absorbents; lightstabilizers; fluorescent materials and fluorescent brighteners, to theextent that they do not impair the performance of the cover.

The amount of white pigment (titanium oxide) per 100 parts by mass ofthe resin component in the cover is preferably not less than 0.5 partsby mass, and more preferably not less than 1 part by mass, whereas it ispreferably not more than 10 parts by mass, and more preferably not morethan 8 parts by mass. When the amount of white pigment is not less than0.5 parts by mass, hiding properties can be imparted to the cover. Ifthe amount of white pigment exceeds 10 parts by mass, the durability ofthe resulting cover may be reduced.

The cover preferably has a slab hardness (Hc) of not higher than 50,more preferably not higher than 40, and still more preferably not higherthan 35 in Shore D hardness. The cover with a slab hardness of nothigher than 50 improves the spin performance on approach shots with ashort iron or the like, thus enabling to produce a golf ball excellentin controllability on approach shots. The cover preferably has a slabhardness (Hc) of not lower than 10, more preferably not lower than 15 inShore D hardness. The cover with a slab hardness lower than 10 mayexcessively increase the spin rate on approach shots with a short ironor the like. The slab hardness of the cover herein refers to thehardness of the cover composition in a sheet form measured by the methodmentioned later.

The embodiment of forming the cover from the cover composition is notparticularly limited. Examples thereof include: an embodiment in whichthe cover composition is injection-molded directly onto a core; and anembodiment in which the cover composition is molded into hollow shells,and a core is covered with a plurality of shells and thencompression-molded (preferably, a method in which the cover compositionis molded into hollow half-shells, and a core is covered with two piecesof half-shells and then compression-molded). In the case of injectionmolding the cover composition onto a core to form a cover, upper andlower molds for forming a cover each preferably have a hemisphericalcavity with pimples, some of the pimples also serving as retractablehold pins. The cover can be formed by injection molding as follows: thehold pins are protruded; a core is placed in the mold and held by thehold pins; then, the cover composition melted by heating is injectedonto the core and then cooled to form a cover. For example, the covercomposition melted by heating to 150 to 230° C. is injected in 0.1 to 1second into molds clamped under a pressure of 980 KPa to 1,500 KPa, thecomposition is then cooled for 15 to 60 seconds, and the molds areopened to obtain a cover.

In the case that the cover is formed by compression molding, half-shellscan be formed by either compression molding or injection molding,suitably by compression molding. The conditions for compression moldingthe cover composition into half-shells may be, for example, under apressure of at least 1 MPa but not more than 20 MPa at a moldingtemperature of at least −20° C. but not higher than +70° C. with respectto the flow beginning temperature of the cover composition. Under thesemolding conditions, half-shells having a uniform thickness can beformed. In an example of the method for forming a cover from thehalf-shells, a core is covered with two pieces of half-shells and theyare subjected to compression molding. The conditions for compressionmolding the half-shells to form a cover may be, for example, under amolding pressure of at least 0.5 MPa but not more than 25 MPa at amolding temperature of at least −20° C. but not higher than +70° C. withrespect to the flow beginning temperature of the cover composition.Under these molding conditions, a cover for golf balls having a uniformthickness can be formed.

When a golf ball body is formed by covering with the cover, the surfaceof the cover typically has dents called dimples. The total number ofdimples formed on the cover is preferably 200 to 500. If the number ofdimples is less than 200, the effect of dimples is less likely to beexhibited. If the number of dimples is more than 500, the individualsize of dimples is small and thus the effect of dimples is less likelyto be exhibited. The dimples formed each may have any shape (shape inplane view), and may have a round shape; a polygonal shape such assubstantially triangle, substantially rectangle, substantially pentagon,or substantially hexagon; or other irregular shapes. These shapes may beemployed alone, or two or more of these shapes may be employed incombination, for the shapes of dimples.

Moreover, the golf ball body with the thus formed cover is taken outfrom the mold and may then preferably be subjected to a surfacetreatment such as deburring, cleaning, and sandblasting as needed. Ifdesired, a paint layer or a mark may be formed. The thickness of thepaint layer is not particularly limited, and is preferably not smallerthan 5 μm, and more preferably not smaller than 7 μm, whereas it ispreferably not larger than 25 μm, and more preferably not larger than 23μm. If the thickness of the paint layer is smaller than 5 μm, the paintlayer is more likely to wear out and disappear after continuous use,while if the thickness of the paint layer is larger than 25 μm, theeffect of dimples tends to decrease and thus the resulting golf balltends to have reduced flight performance.

The cover of the golf ball of the present invention preferably has athickness of not smaller than 0.3 mm, and more preferably not smallerthan 0.4 mm. A thinner cover enables a higher initial speed. The coverpreferably has a thickness of not larger than 1.5 mm, and morepreferably not larger than 1.0 mm. A thicker cover improves spinperformance but may lower the initial speed. The thickness of the coverherein refers to the thickness of portions of the cover without dimples.In other words, the thickness is determined by measuring the thicknessof the cover at at least four points beneath a land portion, andcalculating the mean value.

The golf ball of the present invention may have any structure as long asit includes a core, at least one middle layer covering the core, and acover covering the middle layer. Specific examples of the structure ofthe golf ball of the present invention include: a three-piece golf ballthat includes a core, a middle layer covering the core, and a covercovering the middle layer; a four-piece golf ball that includes a coreincluding a center and a surrounding layer covering the center, a middlelayer covering the core, and a cover covering the middle layer; and amulti-piece golf ball that includes a core including a center and asurrounding layer covering the center, multiple pieces or layers ofmiddle layer covering the core, and a cover covering the middle layer.

In cases where the golf ball of the present invention has a diameter of40 to 45 mm, the amount of compressive deformation (shrinkage of thegolf ball in the compression direction) when applying from an initialload of 98 N to a final load of 1275 N is preferably not smaller than2.0 mm, more preferably not smaller than 2.1 mm, and still morepreferably not smaller than 2.2 mm, whereas it is preferably not largerthan 3.0 mm, more preferably not larger than 2.9 mm, and still morepreferably not larger than 2.8 mm. When the amount of compressivedeformation is not smaller than 2.0 mm, better shot feeling can beprovided. When the amount of compressive deformation is not greater than3.0 mm, good resilience can be achieved.

EXAMPLES

The present invention will be described in greater detail referring to,but not limited to, examples.

(1) Surface Hardness of Center, Surface Hardness of Surrounding Layer(Shore D Hardness)

Using a P1-series auto rubber hardness tester (produced by KobunshiKeiki Co., Ltd.) including a spring type Shore D hardness tester inaccordance with ASTM-D 2240, the Shore D hardness was measured at thesurface of a center and at the surface of a surrounding layer, and usedas the surface hardness of the center and the surface hardness of thesurrounding layer, respectively.

(2) Slab Hardness (Shore D Hardness)

A material for a middle layer or a cover composition was formed into asheet having a thickness of about 2 mm, and then stored at 23° C. for 2weeks. Three or more pieces of this sheet were stacked on one another soas not to be affected by a measurement substrate and the like. The slabhardness of the stack was measured using a P1-series auto rubberhardness tester (produced by Kobunshi Keiki Co., Ltd.) including aspring type Shore D hardness tester in accordance with ASTM-D 2240. Thesheet used in the measurement was formed by injection molding.

(3) Upper Yield Stress, Lower Yield Stress (MPa)

A material for a middle layer was injection-molded into a sheet having athickness of about 2 mm, and then stored at a temperature of 23° C. for2 weeks. A dumbbell specimen was prepared from this sheet. The specimenwas subjected to a tensile test in accordance with ISO 527-1 to preparea stress-strain curve. A value at a point on the curve at which thestress first started declining due to the increase in strain was takenas upper yield stress. A value at a point on the curve at which thestress first started rising after passing through the upper yield stressdue to the increase in strain was taken as lower yield stress.

(4) Breaking Stress (MPa)

A material for a middle layer was injection-molded into a sheet having athickness of about 2 mm, and then stored at a temperature of 23° C. for2 weeks. A dumbbell specimen was prepared from this sheet. The specimenwas measured for breaking stress in accordance with ISO 527-1.

(5) Bending Rigidity (MPa)

A specimen (length: 80.0±2 mm, width: 10.0±0.2 mm, thickness: 4.0±0.2mm) was prepared by injection molding a material for a middle layer, andthen stored at a temperature of 23° C. for 2 weeks. Then, the bendingrigidity of the specimen sheet was measured in accordance with ISO 178.The measurement was performed at a temperature of 23° C. and a humidityof 50% RH.

(6) Amount of Compressive Deformation (mm)

A golf ball was compressed by applying from an initial load of 98 N to afinal load of 1275 N to the golf ball, and the amount of deformation ofthe golf ball in the compression direction (shrinkage of the golf ballin the compression direction) was measured.

(7) Driver Shots

A golf ball was hit at a clubhead speed of 50 m/sec with a metal headdriver (W #1) (XXIO S, loft angle: 11°, produced by Dunlop Sports Co.,Ltd.) attached to a swing robot M/C (produced by Golf LaboratoriesInc.). The flight distance (the distance from a launching point to astopping point), the initial speed of the ball, and the spin rate (therate of spin of the ball) were measured. Each golf ball was measured 12times, and a mean value was calculated and used as the measured value ofthe golf ball. It should be noted that the spin rate of a golf ballimmediately after being hit was determined from serial photographs ofthe golf ball hit.

(8) Durability

Each golf ball was hit at a clubhead speed of 50 m/sec with a metal headdriver (W #1) attached to a swing robot M/C (produced by GolfLaboratories Inc.) to make the golf ball collide with a collision board.This operation was repeated. The number of hits required to break thegolf ball was measured. The number of hits for each golf ball isexpressed as an index relative to that of a golf ball No. 12 (=100), toshow the durability of the golf ball. A higher index indicates that thegolf ball has better durability at high clubhead speeds.

[Preparation of Golf Ball] (1) Preparation of Center

A rubber composition was prepared by mixing the materials according tothe formulation shown in Table 1. The rubber composition was hot-pressedin upper and lower molds each having a hemispherical cavity at 170° C.for 30 minutes to form a center.

TABLE 1 Composition for center No. A Formulation Polybutadiene rubber100 (Parts by Zinc acrylate 31.5 mass) Zinc oxide 5 Barium sulfateAppropriate amount Diphenyl disulfide 0.3 Dicumyl peroxide 0.9 PhysicalSurface hardness 60 properties (Shore D hardness) Polybutadiene rubber:“BR-730 (high-cis polybutadiene)” produced by JSR Corporation Zincacrylate: “ZNDA-90S” produced by Ninon Jyoryu Kogyo Co., Ltd. Zincoxide: “GINREI (registered trademark) R” produced by Toho Zinc Co., Ltd.Barium sulfate: “Barium Sulfate BD” produced by Sakai Chemical IndustryCo., Ltd. Diphenyl disulfide: product of Sumitomo Seika Chemicals Co.,Ltd. Dicumyl peroxide: “PERCUMYL (registered trademark) D” produced byNOF Corporation

The amount of barium sulfate added was appropriately adjusted so thatthe resulting golf ball had a mass of 45.4 g.

(2) Preparation of Composition for Surrounding Layer

A composition for a surrounding layer was prepared using the materialsaccording to the formulation shown in Table 2.

TABLE 2 Composition for surrounding layer No. B FormulationPolybutadiene rubber 100 (Parts by Zinc acrylate 38 mass) Zinc oxide 5Barium sulfate Appropriate amount Diphenyl disulfide 0.5 Dicumylperoxide 0.8 Physical Surface hardness 62 properties (Shore D hardness)Polybutadiene rubber: “BR-730 (high-cis polybutadiene)” produced by JSRCorporation Zinc acrylate: “ZNDA-90S” produced by Nihon Jyoryu KogyoCo., Ltd. Zinc oxide: “GINREI (registered trademark) R” produced by TohoZinc Co., Ltd. Barium sulfate: “Barium Sulfate BD” produced by SakaiChemical Industry Co., Ltd. Diphenyl disulfide: product of SumitomoSeika Chemicals Co., Ltd. Dicumyl peroxide: “PERCUMYL (registeredtrademark) D” produced by NOF Corporation

(3) Preparation of Cover Composition and Material for Middle Layer

A pelletized cover composition and a pelletized material for a middlelayer were prepared by mixing the materials according to theformulations shown in Table 3 and Table 4 using a twin-screw mixingextruder. The following extrusion conditions were used: a screw diameterof 45 mm, a screw rotation speed of 200 rpm, and a screw L/D ratio of35. Here, the mixture was heated to 160 to 230° C. in the die of theextruder.

TABLE 3 Cover composition No. 1 2 3 Formulation Elastollan XNY75A 100 —— (Parts by Elastollan XNY83A — 100 — mass) Elastollan XNY85A — — 100Titanium oxide  4  4  4 Physical Slab hardness  23  30  32 properties(Shore D hardness) Elastollan XNY75A: thermoplastic polyurethaneelastomer (Shore D hardness: 23) produced by BASF Elastollan XNY83A:thermoplastic polyurethane elastomer (Shore D hardness: 30) produced byBASF Elastollan XNY85A: thermoplastic polyurethane elastomer (Shore Dhardness: 32) produced by BASF

TABLE 4 Material for middle layer No. a b c d e g f FormulationPrimalloy CX300 100 — — — — — — (Parts by (Polyester elastomer) mass)Elastollan 1164D — 50 — — — — — (Polyether polyurethane elastomer)Elastollan 1174D — 50 100 — — — — (Polyether polyurethane elastomer)Elastollan HM76D — — — — — 100 — (Polyether polyurethane elastomer) E568— — — 100 — — — (Polycaprolactone polyurethane elastomer) E574 — — — —100 — — (Polycaprolactone polyurethane elastomer) Surlyn 8945 — — — — —— 50 (Zn ionomer, Acid component content: 15 wt %) Himilan AM7329 — — —— — — 50 (Na ionomer, Acid component content: 15 wt %) Physical Slabhardness (Shore D hardness) 72 70 74 68 74 76 66 properties Bendingrigidity (MPa) 400 360 660 350 640 700 290 of slab Breaking stress (MPa)29 55 55 55 56 50 27 Breaking stress/Slab hardness (Shore D hardness)0.40 0.79 0.74 0.81 0.76 0.66 0.41 Upper yield stress (MPa) 23 25 31 2832 38 18 Lower yield stress (MPa) 14 24 22 25 24 22 17 Upper yieldstress/Lower yield stress 1.64 1.04 1.41 1.12 1.33 1.73 1.06 PRIMALLOYCX300: polyester elastomer (Shore D hardness: 72, bending rigidity: 400MPa, breaking stress: 29 MPa, upper yield stress: 23 MPa, lower yieldstress: 14 MPa) produced by Mitsubishi Chemical Corporation Elastollan1164D: polyether polyurethane elastomer (Shore D hardness: 64, bendingrigidity: 330 MPa, breaking stress: 55 MPa, upper yield stress: 23 MPa,lower yield stress: 23 MPa) produced by BASF Elastollan 1174D: polyetherpolyurethane elastomer (Shore D hardness: 74, bending rigidity: 660 MPa,breaking stress: 55 MPa, upper yield stress: 31 MPa, lower yield stress:22 MPa) produced by BASF Elastollan HM76D: polyether polyurethaneelastomer (Shore D hardness: 76, bending rigidity: 700 MPa, breakingstress: 50 MPa, upper yield stress: 38 MPa, lower yield stress: 22 MPa)produced by BASF Miractran E568: thermoplastic polycaprolactonepolyurethane elastomer (Shore D hardness: 68, bending rigidity: 350 MPa,breaking stress: 55 MPa, upper yield stress: 28 MPa, lower yield stress:25 MPa) produced by Nippon Miractran Co, Ltd. Miractran E574:thermoplastic polycaprolactone polyurethane elastomer (Shore D hardness:74, bending rigidity: 640 MPa, breaking stress: 56 MPa, upper yieldstress: 32 MPa, lower yield stress: 24 MPa) produced by Nippon MiractranCo, Ltd. Surlyn 8945: sodium ion neutralized ethylene-methacrylic acidcopolymer ionomer resin (acid component content: 15% by mass, Shore Dhardness: 61, bending rigidity: 254 MPa, breaking stress: 24 MPa, upperyield stress: 15 MPa, lower yield stress: 14 MPa) produced by E. I. duPont de Nemours and Company Himilan AM7329: zinc ion neutralizedethylene-methacrylic acid copolymer ionomer resin (acid componentcontent: 15% by mass, Shore D hardness: 61, bending rigidity: 254 MPa,breaking stress: 24 MPa, upper yield stress: 16 MPa, lower yield stress:14 MPa) produced by Du Pont-Mitsui Polychemical

(4) Preparation of Golf Ball Body

The above-obtained composition for a surrounding layer was used to forma surrounding layer on the center obtained as above, if necessary, and acore was thus prepared. In the case of using the composition for asurrounding layer, first the composition for a surrounding layercontaining the materials according to the formulation shown in Table 2was kneaded, and then an upper mold for forming a core, in which thecenter was housed, and a lower mold for forming a core were clamped in amanner that a necessary amount of the composition for a surroundinglayer was in contact with a half of the surface of the center, followedby pressing. Thus, an intermediate core molded product in which asurrounding layer was formed on a half of the surface of the center wasprepared. Next, a lower mold for forming a core, in which thesurrounding layer of the intermediate core molded product was housed,and an upper mold for forming a core were clamped in a manner that anecessary amount of the composition for a surrounding layer was incontact with the other half of the surface of the center, followed bypressing. Thus, a surrounding layer was formed on the other half of thesurface of the center, and the resulting product was hot-pressed at 170°C. for 30 minutes to form a core.

The material for a middle layer obtained as above was injection-moldedonto the core obtained as above to form a middle layer covering thecore. Then, a cover was formed by injection molding the covercomposition onto the middle layer. Or alternatively, a cover was formedby injection molding or compression molding the cover composition intohalf-shells and applying two pieces of half-shells to the core providedwith the middle layer so as to cover the core, followed by hot-pressing.Thus, a golf ball was prepared. The upper and lower molds for moldingused each had a hemispherical cavity with pimples, some of the pimplesalso serving as retractable hold pins. After protruding the hold pins,the core was placed in the mold and held by the hold pins, and the moldswere clamped under a pressure of 80 t. Then, the resin compositionheated to 210° C. was injected into the molds in 0.3 seconds, and thencooled for 30 seconds. The molds were opened to take out a golf ball.

The surface of the obtained golf ball body was sandblasted and marked.Then, a clear paint was applied and dried by heating in an oven at 40°C. for 4 hours. Thus, a golf ball having a diameter of 42.7 mm and amass of 45.4 g was obtained.

The golf balls thus obtained were evaluated for amount of compressivedeformation and other items. Table 5 shows the results.

TABLE 5 Golf ball No. 1 2 3 4 5 6 7 8 9 10 Structure 3PC 3PC 3PC 3PC 3PC3PC 3PC 3PC 4PC 4PC Core Composition for A A A A A A A A A A center No.Composition for — — — — — — — — B B surrounding layer No. MiddleComposition for b c d e b b b b b c layer middle layer No. Cover Cover 22 2 2 1 3 2 2 2 2 composition No. Details of Core diameter 39.7 39.739.7 39.7 39.7 39.7 40.1 38.1 39.7 39.7 structure (mm) (Center (Centerdiameter: diameter: 20.1 mm) 20.1 mm) (Surrounding (Surrounding layerlayer thickness: thickness: 9.8 mm) 9.8 mm) Middle layer 1.0 1.0 1.0 1.01.0 1.0 0.8 1.8 1.0 1.0 thickness (mm) Cover 0.5 0.5 0.5 0.5 0.5 0.5 0.50.5 0.5 0.5 thickness (mm) Ball diameter 42.7 42.7 42.7 42.7 42.7 42.742.7 42.7 42.7 42.7 (mm) Physical Amount of 2.68 2.66 2.68 2.66 2.692.65 2.71 2.62 2.62 2.60 properties compressive of golf deformation ball(mm) Ball initial 72.6 72.6 72.6 72.7 72.6 72.7 72.8 72.7 72.7 72.8speed (m/s) Ball spin 2375 2360 2395 2335 2400 2360 2380 2340 2350 2340rate (rpm) Flight 252.8 253.1 252.5 253.8 252.1 253.4 253.2 253.6 253.5253.7 distance (m) Durability 160 125 140 125 165 155 130 170 145 120(index) Golf ball No. 11 12 13 14 15 16 17 18 19 Structure 3PC 3PC 3PC3PC 3PC 3PC 3PC 4PC 4PC Core Composition for A A A A A A A A A centerNo. Composition for — — — — — — — B B surrounding layer No. MiddleComposition for b b a f g a a a f layer middle layer No. Cover Cover 2 22 2 2 1 3 2 2 composition No. Details of Core diameter 41.1 36.7 39.739.7 39.7 39.7 39.7 39.7 39.7 structure (mm) (Center (Center diameter:diameter: 20.1 mm) 20.1 mm) (Surrounding (Surrounding layer layerthickness: thickness: 9.8 mm) 9.8 mm) Middle layer 0.3 2.5 1.0 1.0 1.01.0 1.0 1.0 1.0 thickness (mm) Cover 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5thickness (mm) Ball diameter 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.742.7 (mm) Physical Amount of 2.74 2.56 2.70 2.72 2.64 2.73 2.67 2.632.65 properties compressive of golf deformation ball (mm) Ball initial73.0 72.6 72.5 72.6 Not 72.4 72.5 72.6 72.7 speed (m/s) evaluable Ballspin 2430 2320 2475 2450 due to 2500 2445 2450 2430 rate (rpm) poorFlight 253.6 252.8 250.6 251.1 durability 249.9 251.2 251.4 251.6distance (m) Durability 105 180 25 100 30 20 10 90 (index)

It was demonstrated that each of the golf balls Nos. 1 to 10, whichincluded a middle layer including a thermoplastic polyurethane elastomerhaving a specific slab hardness and a specific ratio of upper yieldstress to lower yield stress, showed a lower spin rate, longer flightdistance and better durability than the golf balls Nos. 13, 14, and 16to 19, which included a polyester elastomer or an ionomer resin.Further, excellent flight distance and excellent durability were alsoensured in the case of the golf balls No. 11 and 12 including a middlelayer with a thickness of 0.3 mm and 2.5 mm, respectively. The golf ballNo. 15 had poor durability and thus was not evaluable.

INDUSTRIAL APPLICABILITY

The golf ball of the present invention is useful as it has improvedflight distance performance and durability.

1. A golf ball, comprising a core, at least one middle layer covering the core, and a cover covering the middle layer, wherein at least one piece or layer of the middle layer comprises a material for a middle layer that comprises a thermoplastic polyurethane with a slab hardness of 65 to 80 in Shore D hardness and a ratio of upper yield stress (MPa) to lower yield stress (MPa) of not more than 1.60.
 2. The golf ball according to claim 1, wherein the thermoplastic polyurethane has an upper yield stress of not less than 15 MPa and a lower yield stress of not less than 10 MPa.
 3. The golf ball according to claim 1, wherein the thermoplastic polyurethane has a breaking stress (MPa) of not less than 25 MPa.
 4. The golf ball according to claim 1, wherein the thermoplastic polyurethane has a ratio of breaking stress (MPa) to slab hardness (Shore D hardness) of not less than 0.70.
 5. The golf ball according to claim 1, wherein the thermoplastic polyurethane has a bending rigidity of 250 to 4000 MPa.
 6. The golf ball according to claim 1, wherein the middle layer has a thickness of 0.5 to 2.0 mm.
 7. The golf ball according to claim 1, wherein the middle layer has a surface hardness of 65 to 80 in Shore D hardness.
 8. The golf ball according to claim 1, wherein a difference (Hm−Hs) between a surface hardness (Hm) of the middle layer and a surface hardness (Hs) of the core is 3 to
 25. 9. The golf ball according to claim 1, wherein the cover comprises a thermoplastic polyurethane with a slab hardness of not more than 50 in Shore D hardness, and has a thickness of 0.3 to 1.5 mm. 